If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I fried some components some time ago on my inverter. I measured the capacitance of each metal film capacitor and they seemed ok. Yet, when I ran them at 100khz it seemed that they were partially shorting, and causing my inverter voltage to sag heavily. I replaced the capacitors and everything worked perfectly.
So, if the capacitance is good under static conditions, how can one tell the dielectric is failing? Their rating is 0.22uf, so I would think timing how long they hold a charge would be difficult.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Where I worked we bought one of these :
We tried to use it to analyse induction heater oil capacitors to see if failures could be predicted in a Calcoil machine.
It didnt.
Then we built a High voltage DC hypot leakage tester and picked out a few dodgy ones from the modules as they came for repair.
Finally after gathering a box of about 100 bad capacitors we complained so loudly the capacitor manufacturer admtited they had a bad lot and replaced them all
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
There is another test done on very large motors to determine a 'polarization index' which measures how much water the machine sucked up while it was switched off. These tests are a part of startup after a plant has been down for extended maintenance. It very well may be that the seal broached on the small film capacitors allowing moisture to get in. They might then present a different leakage depending on polarity. A HP valve VTVM has a 100 meg DC input and can show if a capacitor shows electret properties. A thermal imaging camera also could show which of a group were getting hotter during operation.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Film or film/foil capacitors can die slowly as internal faults get vapourised, causing a drop in capacitance. Since you say the capacitance value measured is correct it isn't that.
Leakage current should be virtually un-measurable, check a data sheet for your type of capacitor.
If the capacitor has been subjected to large impulse or rms currents the wire-to-foil/film connection (schoopage) can degrade, increasing esr, which is difficult to measure simply. The result will be overheating.
Possibly you are over-volting the capacitor(s) due to resonance etc. ? I would expect to see a slow loss of capacitance in this case.
For almost all film capacitors, if it feels hot then you're over-stressing it.
P.S. Have you looked at a datasheet for your capacitor or similar type? You will find that the permissible rms voltage at 100 kHz is MUCH less than at 60 Hz.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
How can you test for a bad capacitor
